This invention relates generally to an imaging system for projecting an image of an object lying on an object plane, such as a transparent platen, onto an image plane and, more particularly, to an improved system which incorporates a three-dimensional corrector element positioned along an optical path to correct for cos 4 illumination falloff and other illumination non-uniformities of the system projection lens so as to provide a uniform exposure level at the image plane.
In imaging systems such as those used in electrophotographic copying and printing devices, a document is placed on a transparent platen surface, illuminated by a scanning or flash illumination assembly, and a reflected image of the document is projected through a lens onto the charged surface of a photosensitive member such as a xerographic drum or belt-type photoreceptor. Those areas of the charged surface which are irradiated by the projected image are discharged, forming a latent image of the document. The degree of discharge is dependent on the intensity of the projected light rays. It is important that the exposure irradiance at the charged photosensitive surface vary only due to the reflectance characteristics of the original document being copied rather than due to changes introduced by the imaging components such as the lens and folding mirrors. Stated in another manner, an optimum system would provide uniform photoreceptor irradiance given a uniformly reflecting document.
The most significant factor which affects the relative illumination at an image plane is the cosine (cos 4) variation caused by the projection lens viewing document points at the outer ends of the field of view. The illumination at the image plane is proportional to the cos 4 of the angle between the optical axis and the off-axis principal rays. Irradiance at the image plane therefore decreases as radial distance from the system optical axis increases, resulting in a lower level of exposure of the edges of the document thereby causing resulting loss of image quality in copies made from the developed image.
Various approaches have been devised to compensate for this cos 4 effect. Typically, in scanning systems, a sheet of opaque material having a butterfly slit formed thereon is placed in the optical path near the object or image plane thereby acting as a field stop. The area of the slit is inversely proportional to the illumination profile. Other similar techniques employ masks having slits of this nature integral with the lens. Still other scanning systems place a variable density filter in the optical path, the transmissiveness of the filter varying inversely to the cos 4 angle.
It is also known to use optical stops in photographic systems to reduce oblique spherical aberrations. A system employing optical stops is disclosed in U.S. Pat. No. 3,504,960. This type of stop is characterized by having borders of various configurations surrounding a central clear aperture. These stops are not, however, concerned with, and are not a substitute for, cos 4 compensation.
Another prior art imaging system disclosed in U.S. Pat. No. 4,445,774, assigned to the same assignee as the present application, utilizes an opaque illumination corrector of a specially designed shape in the optical path to selectively compensate for the cos 4 effects.
In full frame imaging systems where an entire document is typically illuminated by a flash lamp, efforts to compensate for cos 4 light falloff have emphasized locating the lamps so that the document edges are illuminated to a greater degree than central areas. Two such systems are disclosed in U.S. Pat. Nos. 3,669,538 and 3,777,135. Another related technique is to place a variable density filter into the optical path as disclosed in U.S. Pat. No. 4,298,274.
The above-described compensation techniques each have significant disadvantages. The variable density filters are relatively expensive and their use entails significant energy losses. The optical slit techniques also entail energy losses and require substantial modification of the imaging system so as to operate in more than one magnification reproduction mode. The opaque illuminator corrector disclosed in the '774 patent loses efficiency at the wider field angles and experiences some loss of image quality at an on-axis location.
The present invention, therefore, is directed to a cos 4 relative illumination corrector which is inexpensive and easily placed into the optical path. More particularly, the present invention is directed to an imaging system wherein an object lying in an object plane is illuminated and an image is projected onto an image plane comprising, in combination:
a projection lens, and
at least one three-dimensional corrector positioned between said object and image planes and within the field of view of the lens, said corrector adapted to selectively vignette energy emanating from said field of view so as to provide a relatively uniform exposure level at the image plane.
In a second embodiment, the corrector has a hollow circular interior of varying diameter to improve on-axis image quality while correcting cos 4 falloff.